RF ion guides, such as RF multipoles, are widely used in mass spectrometry in ion optical devices. Examples of devices comprising RF ion guides include mass filters, collision cells, ion traps and transport multipoles. However, the design of such devices presents numerous challenges.
In recent years, RF ion guides with additional axial fields have been developed for better control of ion motion, for example as disclosed in U.S. Pat. No. 5,847,386, U.S. Pat. No. 6,111,250, EP1271611, U.S. Pat. No. 6,674,071, U.S. Pat. No. 6,107,628, U.S. Pat. No. 7,164,125, U.S. Pat. No. 7,064,322 and U.S. Pat. No. 7,564,025. These ion guides typically consist of massive metal or resistive RF rods, usually of circular section, and different means to provide an additional DC distribution along the axis of the guide. Such constructions are typically formed using conventional machining methods and invariably require cumbersome assembly and numerous parts.
In order to simplify the manufacturing of RF ion guides, planar designs have been developed like those described in U.S. Pat. No. 5,572,035, U.S. Pat. No. 6,040,575, U.S. Pat. No. 7,365,317, U.S. Pat. No. 7,786,435, WO2010/014077, U.S. Pat. No. 6,872,941, US2011/240850, WO2006/059123 and WO2004/021385. The latter prior art design is suitable for manufacturing by standard mass-production techniques of electronics, e.g. in the form of printed circuit boards (PCBs), or by lithography for miniaturized designs. Such concepts have been adopted for the production of miniature ion traps for quantum computing (see e.g. Chiaverini et al, Quantum Inform. And Computation, v. 5, No. 6 (2005) 419-439, as well as Kielpinski et al, Nature Vol. 417, 2002, p. 709). However, this approach has the disadvantage of not providing an effective way to construct an ion guide with a deep enough potential well for use in mass spectrometry. Other disadvantages of the foregoing designs include not providing a robust construction that could also be used to confine gas for collisional cooling of ions for example, and charging up of dielectric gaps between electrodes that can affect ion motion. Whilst there are designs without dielectric (e.g. using resistive rods to generate a gradient field), such designs are difficult to manufacture. For conventional multipoles positioned near to the ion source, a large amount of neutral species also typically collide with the multipole rods, which again can affect performance over time.
The invention has been made against this background in order to alleviate one or more of the aforementioned problems.